Mounting into a Rexhall Steel Frame

I have a 30' 1997 airbus and I'm going to mount a solar panel in the roof to trickle charge my battery bank. I know the Rexhall frames are steel, but does anyone have experience in mounting into them? I mainly want to know if I can just use a self tapping screw (tapcon), or is it pretty thick and I need to drill in and tap a hole? Do you think I could just use an expansion nut into and through the Fiberglass RV roof for the mounting of the panel?
Any ideas on the easiest way to thread the wiring through the RV and down to the Battery area would be helpful too.